The art of electrocoating has been emerging for some time and has now become an important method of providing industrial decorative and protective finishes. The coatings achieved have excellent properties for many applications and electrodeposition results in a coating which does not run or wash off during baking. Virtually any conductive substrate may be coated by electrodeposition. The most commonly employed substrates include the base metals such as iron, steel, aluminum, copper, zinc, brass, tin, nickel and chromium, as well as other metals and pretreated metals; impregnated paper or other substrates rendered conductive under the conditions employed may also be coated.
Since the advent of electrocoating, organic polymer researchers have intensely investigated various means by which stability may be both maintained and controlled. For example, U.S. Pat. No. 3,496,083 employs an improved method for forming a coating film in which cloth of vegetable fibers is used as a diaphragm to remove alkaline cations into a cathode compartment, and then removing the alkaline cations to outside the electrodeposition bath. U.S. Pat. No. 3,419,488 employs an electrode which is separated from the aqueous dispersion or solution by an ion-exchange membrane selectively permeable to ions attracted to the counter-electrode. Both of these patents maintain stability by purging, from the bath, excess ion buildup. U.S. Pat. No. 3,576,728 provides for the preparation of a stable emulsion by mixing an insoluble non-ionizable organic material (acid value below 15), a polar dispersing agent, and alkaline water. These emulsions are useful for coating self-terminating polymer films on conducting substrates. The electrodeposition baths can be replenished from a concentrate material without any disproportionate buildup of bath components.
These methods, while useful in many cases, are not always satisfactory because when an ionizable organic resin, which has been neutralized with a metal hydroxide, is employed, even the use of fiber diaphragms and ion exchange membranes are often not sufficient to maintain bath stability.